The present invention relates to acousto-optic modulators, and more particularly to a multimode acousto-optic switch that is tailored to the beam characteristics of a multimode collimated light beam to allow the beam to be deflected with acceptable efficiency while providing good optical beam separation and to a method for reducing the dead zone due to the finite rise time of such a switch.
In conventional optical time domain reflectometry (OTDR) optical pulses are launched into an optical fiber to be tested, and the backscattered radiation is detected and displayed as a function of position on the fiber. Features of the detected data include locations of splices and breaks as well as the Rayleigh backscatter from microscopic inhomogeneities in the fiber. Detection of the Rayleigh backscatter is very important since it allows a single ended measurement of splice loss as well as a means of verifying that the fiber is actually attached. The need to detect the Rayleigh backscatter, which is typically 40-50 dB weaker than Fresnel reflections, requires that OTDR optics launch and receive optical power efficiently, i.e., low insertion loss, and that weak signals be readily measured in proximity to large reflections. Large signals produce a detector "tail", a relatively long time interval during which the optical detector recovers to its unilluminated signal level. During this tail interval the detector output is sufficiently large to obscure detection of the weak Rayleigh backscatter, or at least to reduce the accuracy of the light measurement.
A successful technique proposed to mask the detector from the reflective features that produce the large signals has been to use an acousto-optic modulator, i.e., a Bragg cell, for single mode optical fibers where a Gaussian approximation to the optical mode profile is appropriate. In the Bragg cell an acousto-optic medium, such as TeO.sub.2, is used to deflect the incoming light beam from the fiber to one of two ports, depending upon launch and receiving conditions. This technique is described by Horiguchi et al in the Journal of Lightwave Technology, Vol. LT-2, No. 2, April 1984, in an article entitled "An Acoustooptical Directional Coupler for an Optical Time-Domain Reflectometer." However multimode fiber operation is more problematic since the fiber output beam is much larger making collimation and Bragg diffraction more difficult.
Also there is a dead zone during the rise time of the optical switch which has a lower limit determined by the combined speed of response of the switch and the receiver system. This lower limit may be as large as 300 nsec or 30 meters length of fiber.
Therefore what is desired is an acousto-optic modulator that is tailored to the beam characteristics of a multimode beam from a multimode fiber optic collimator that allows the beam to be deflected with acceptable efficiency while providing good optical beam separation while providing a method of correcting for the dead zone associated with the rise time of the switch and receiver system.